Causes of Preterm Labor

If you are at risk for preterm labor, several screening tests can help you and your doctor determine the extent of your risk. These tests measure changes that indicate the onset of labor and changes that are associated with an increased risk of preterm labor. These tests can be performed before you have any signs of preterm labor or they may be used after labor has begun.

When a baby is born before the 37th week of pregnancy, it is called a preterm delivery. Some preterm births happen on their own -- a mother goes into labor and her baby comes early. In other cases, problems with the pregnancy prompt doctors to deliver a baby earlier than planned. About three quarters of preterm births are spontaneous and about one quarter occur because of medical complications. Overall, about one in eight pregnant women deliver early.

SCREENING TEST

WHAT THE TEST DETECTS

Transvaginal ultrasound

shortening and dilation (opening) of the cervix

Uterine monitoring

uterine contractions

Fetal fibronectin

chemical changes in the lower uterus

Testing for vaginal infections

bacterial vaginosis (BV)

Doctors are not yet sure how many tests-or which combination of tests-are most helpful in determining the risk for preterm labor. This is still being studied. They do know, however, that the more screening tests a woman is positive for, the higher her risk for preterm delivery. For example, if a woman is in her 24th week of pregnancy with no history of preterm labor and no current symptoms of labor, her cervical ultrasound shows that her cervix is over 3.5 cm in length, and her fetal fibronectin is negative, she has a less than one percent chance of delivering before her 32nd week. However, if the same woman has a history of preterm delivery, a positive fetal fibronectin test, and her cervix measures less than 2.5 cm in length, she has a 50% chance of delivering before her 32nd week.

Causes of Preterm Delivery

Preterm delivery has several causes. Sometimes a woman goes into labor early for no obvious reason. At other times there may be a medical reason for early labor and delivery. The chart below lists the causes of preterm delivery and the percentages of women who deliver early due to each cause. In this chart, the category ?preterm labor? refers to women who have no known reason for early labor and delivery.

CAUSE OF PRETERM DELIVERY

PERCENTAGE OF WOMEN WHO DELIVER EARLY

Premature rupture of membranes

30%

Preterm labor (no known cause)

25%

Bleeding during pregnancy (antepartum hemorrhage)

20%

Hypertensive disorders of pregnancy

14%

Weak cervix (incompetent cervix)

9%

Other

2%

Why Is Preterm Labor a Serious Problem?

Despite remarkable medical advances in the care of preterm babies, the environment of a mother's womb cannot be matched. Each week that a fetus remains in the womb increases the chances of survival. For example:

A fetus born before 23 weeks cannot survive outside the mother's womb.

The fetus' ability to survive outside the womb increases dramatically between 24 and 28 weeks, from about 50 percent at the beginning of the 24th week to more than 80 percent four weeks later.

After 28 weeks of pregnancy, more than 90 percent of babies can survive on their own.

There is also a relationship between a baby's gestational age at birth and the likelihood that he or she will have complications after birth. For example:

Babies born before 25 weeks have a very high risk of long-term problems, including learning disabilities and neurological problems. About 20 percent of these babies will be severely disabled.

Before the 28th week of pregnancy, almost all babies will have short-term complications, such as difficulty breathing. About 20 percent of babies will also have some long-term problems.

Between the 28th and 32nd weeks of pregnancy, babies gradually improve. After 32 weeks, the risk of long-term problems is less than 10 percent.

After the 37th week of pregnancy, only a small number of babies will have complications (such as jaundice, abnormal glucose levels, or infection), even though they are full term.

According to the March of Dimes, the average hospital stay for a preterm baby costs $57,000, compared with $3,900 for a term baby. Total costs to health insurers topped $4.7 billion in a 1992 study. Despite this dramatic statistic, many advances in technology have allowed very small babies to go home, do well, and grow up to be healthy children.